The present invention relates to a vortex flow meter for measuring a flow rate of a fluid based on Karman vortex streets, or vortex trains.
A vortex flow meter is well known in the flow measurement art in which a vortex shedding member is positioned across a passageway inside a conduit through which a fluid flows. The velocity or flow rate of the fluid in the conduit is measured by sensing pressure variations of the fluid caused by vortex trains. In this kind of flow meter, mechanical and other vibrations attributable to mechanical vibrations of the conduit, turbulent flows of the fluid itself, pulsation of a pump, and the like are apt to appear as errors in an output signal of the flow meter. For the compensation of such errors in measurement, it has been practiced to use a sensor responsive to mechanical vibrations and a sensor responsive to vortex trains and provide a difference between output signals of the two sensors. The problem with such an approach is that the two sensors are constructed integrally with each other but separately from the vortex shedding member and, therefore, have to be assembled independently of the latter. Especially, where the flow meter is small size, difficulty is encountered in machining and assembling it, resulting in an inefficient procedure. Another problem is that since the pressure variation due to a vortex train is proportional to the square of a flow rate, the flow meter needs be responsive to a wide range of pressures and, hence, its sensitivity is very low when flow rate is small. Such has hitherto been coped with by using a preamplifier having a gain which is small for large flow rates and large for small flow rates, i.e. a substantially constant total gain. However, the preamplifier with such a characteristic is susceptive to externally derived vibrations of low frequencies which constitute a source of noise. Attempts have been made to provide a stably operable vortex flow meter by compensating for the external vibrations or allowing a minimum of external vibrations to effect the measurement.